This invention relates to a protective circuit for use in a static inverter. Solid state static inverters are known which are comprised of two groups of semiconductor switching devices, the switching devices of each group being connected in parallel across a source of d.c. voltage. Each switching device has a respective primary winding of an output transformer connected in series therewith. The primary windings of the two groups are wound for push-pull energization of an output transformer. Trigger circuit means are provided for alternately triggering the switching devices of the two groups so as to establish an alternating voltage in the secondary winding of the output transformer. As is well understood, the primary windings associated with one group of conducting switching devices establish a magnetic flux field in one direction in the core of magnetic material of the output transformer and the primary windings associated with the second group of alternately conducting semiconductor devices establish a magnetic flux field in the opposite direction in the core of the output transformer. So long as the two groups of semiconductor devices conduct alternately the transformer core continues to have its magnetization reversed and the current conducted by the semiconductor switching devices will be within desired limits which is a function of the a.c. impedance of the output transformer. However, in the event that one of the groups of switching devices does not receive its trigger pulses, that group of switching devices will not be rendered alternately conductive. As a consequence, the core of the output transformer will be repetitively magnetized in the same direction by the repetitive conduction of the other group of switching devices and the magnetic material will quickly become magnetically saturated. With the core saturated the a.c. impedance of the transformer is significantly reduced and the current flowing through each of the switching devices of that other conducting group quickly may become great enough to damage those switching devices.
Circuits are known which are intended to protect a static inverter against damage which would otherwise result from the type of malfunction discussed above. Some of the known protective circuits employ a current sensing transformer to detect an overcurrent in the inverter output. The sensed overcurrent signal then is employed to initiate operation of the protective circuit. Circuits of this type are useful but suffer the disadvantage that the overcurrent sensing transformer is relatively large and expensive.
Other protective circuits are known but many of them suffer the disadvantages that they require relatively complex circuitry and/or are not fast enough in their operation to protect switching devices other than those having very high current ratings.